Not sure how relevant this is to your situation, but:
https://www.citizensadvice.org.uk/c.../switching-energy-supplier-if-youre-a-tenant/
You can't generally be prevented from changing supplier.
Ah interesting, thanks! Fortunately it looks like my change to a cheaper tariff went through without issue, got an email today from SP to confirm and the IHD for my smart meter is showing the (much) lower rates for units and the standing charge.